Newborn babies are born with around 300 bones, but these are partly or entirely made of cartilage and will be replaced by bone later in life. This is why newborns are so limber, cartilage is more pliable than bone. Bones form and develop over time by a process called endochondral ossification. The cartilage is slowly replaced by calcified bone matrix, it is remodeled by the action of osteoblasts and osteocasts. Calcium salts are slowly laid in the cartilaginous tissue and form hardened bone tissue, this is called calcification of the bone.
A good example of this process is the development of the infant cranium. Newborns have "soft spots" or fontanelles in the skull that slowly grow, calcify, and fuse together to form the hardened bones of the cranium. When the bones eventually fuse together or articulate they form the cranial sutures.
A human neonate, or newborn baby, has around three hundred individual bones and cartilage elements. That sounds like a lot, considering that adults have only two hundred and six bones. As a baby grows into a child and then into a young adult, many of the bones and cartilaginous elements of the body "fuse" or grow together into a single piece. Babies are so flexible because some of their future, bony tissues haven't hardened together yet, and this makes it much easier for the newborn to pass through the mother's birth canal. Some of these bones that fuse together are located in the cranium. If you look at an adult skull, it looks as though a seam runs along the back and sides of the skull. This is where three individual plates of bone have grown and fused together to protect the brain! It's very important that babies heads be protected because there is actually a small portion of the top of the skull that isn't fully protected by bone.
Another part of a baby's body where ossification (bone growth) occurs is the kneecaps. If you look at an x-ray of a baby's legs, it might look like they don't have patellae. They do, but during infancy, the patellae are made of cartilage and so they do not show up on an x-ray. As the child grows, this cartilage will calcify and turn into the sturdy bone all healthy adults have.
A newborn baby has roughly 300 bones. The majority of the "bones" are cartilaginous in nature. This is necessary during fetal development and the birth process to ensure for safe development and delivery during birth a confined space. As the infant develops into a child, then adolescent, and finally an adult, the bones and cartilage continue to fuse and harden into mature skeletal bone. Examples of fusion include the sacrum, coccyx, and sternum. The bones of the skull also harden and fuse into one structure. Final hardening and fusion doesn't occur until eighteen (18) to twenty-five (25) years of age. This is to allow the structures, or organs, in which he bones protect to grow and develop.